Internet Engineering Task Force (IETF) M. Petit-Huguenin
Request for Comments: 7065 Impedance Mismatch
Category: Standards Track S. Nandakumar
ISSN: 2070-1721 G. Salgueiro
P. Jones
Cisco Systems
November 2013
Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers
Abstract
This document specifies the syntax of Uniform Resource Identifier
(URI) schemes for the Traversal Using Relays around NAT (TURN)
protocol. It defines two URI schemes to provision the TURN
Resolution Mechanism (RFC 5928).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7065.
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Petit-Huguenin, et al. Standards Track [Page 1]

RFC 7065 TURN URIs November 20131. Introduction
This document specifies the syntax and semantics of the Uniform
Resource Identifier (URI) scheme for the Traversal Using Relays
around NAT (TURN) protocol.
The TURN protocol is a specification allowing hosts behind NAT to
control the operation of a relay server. The relay server allows
hosts to exchange packets with its peers. The peers themselves may
also be behind NATs. RFC 5766 [RFC5766] defines the specifics of the
TURN protocol.
The "turn" and "turns" URI schemes are used to designate a TURN
server (also known as a relay) on Internet hosts accessible using the
TURN protocol. With the advent of standards such as WebRTC [WEBRTC],
we anticipate a plethora of endpoints and web applications to be able
to identify and communicate with such a TURN server to carry out the
TURN protocol. This implies that endpoints and/or applications must
be provisioned with the appropriate configuration to identify the
TURN server. Having an inconsistent syntax adds ambiguity and can
result in non-interoperable solutions and implementation limitations.
The "turn" and "turns" URI schemes help alleviate most of these
issues by providing a consistent way to describe, configure, and
exchange the information identifying a TURN server.
[RFC5928] defines a resolution mechanism to convert a secure flag, a
host name or IP address, a potentially empty port, and a potentially
empty transport to a list of IP address, port, and TURN transport
tuples.
To simplify the provisioning of TURN clients, this document defines
the "turn" and "turns" URI schemes that can carry the four components
needed for the resolution mechanism.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in [RFC2119] when
they appear in ALL CAPS. When these words are not in ALL CAPS (such
as "should" or "Should"), they have their usual English meanings, and
are not to be interpreted as RFC 2119 key words.
Petit-Huguenin, et al. Standards Track [Page 3]

RFC 7065 TURN URIs November 20133. Definitions of the "turn" and "turns" URI3.1. URI Scheme Syntax
The "turn" and "turns" URIs have the following formal ABNF syntax
[RFC5234]:
turnURI = scheme ":" host [ ":" port ]
[ "?transport=" transport ]
scheme = "turn" / "turns"
transport = "udp" / "tcp" / transport-ext
transport-ext = 1*unreserved
<host> and <port> are specified in [RFC3986]. While these two ABNF
productions are defined in [RFC3986] as components of the generic
hierarchical URI, this does not imply that the "turn" and "turns"
schemes are hierarchical URIs. Developers MUST NOT use a generic
hierarchical URI parser to parse a "turn" or "turns" URI.
The <host>, <port>, and <transport> components are passed without
modification to the [RFC5928] algorithm. <secure> is set to false if
<scheme> is equal to "turn", and set to true if <scheme> is equal to
"turns" and passed to the [RFC5928] algorithm with the other
components.
3.2. URI Scheme Semantics
The "turn" and "turns" URI schemes are used to designate a TURN
server (also known as a relay) on Internet hosts accessible using the
TURN protocol. The TURN protocol supports sending messages over UDP,
TCP, or TLS-over-TCP. The "turns" URI scheme MUST be used when TURN
is run over TLS-over-TCP (or, in the future, DTLS-over-UDP), and the
"turn" scheme MUST be used otherwise.
The required <host> part of the "turn" URI denotes the TURN server
host.
As specified in [RFC5766] and [RFC5928], the <port> part, if present,
denotes the port on which the TURN server is awaiting connection
requests. If it is absent, the default port is 3478 for both UDP and
TCP. The default port for TURN over TLS is 5349.
Petit-Huguenin, et al. Standards Track [Page 4]

RFC 7065 TURN URIs November 20134. Security Considerations
Security considerations for the resolution mechanism are discussed in
Section 5 of [RFC5928]. Note that this section contains normative
text defining authentication procedures to be followed by turn
clients when TLS is used.
The "turn" and "turns" URI schemes do not introduce any specific
security issues beyond the security considerations discussed in
[RFC3986].
Although a "turn" or "turns" URI does not itself include the username
or password that will be used to authenticate the TURN client, in
certain environments, such as WebRTC, the username and password will
almost certainly be provisioned remotely by an external agent at the
same time as a "turns" URI is sent to that client. Thus, in such
situations, if the username and password were received in the clear,
there would be little or no benefit to using a "turns" URI. For this
reason, a TURN client MUST ensure that the username, password,
"turns" URI, and any other security-relevant parameters are received
with equivalent security before using the "turns" URI. Receiving
those parameters over another TLS session can provide the appropriate
level of security, if both TLS sessions are similarly parameterised,
e.g., with commensurate strength ciphersuites.
5. IANA Considerations
This section contains the registration information for the "turn" and
"turns" URI Schemes (in accordance with [RFC4395]).
5.1. "turn" URI Registration
URI scheme name: turn
Status: permanent
URI scheme syntax: See Section 3.1.
URI scheme semantics: See Section 3.2.
Encoding considerations: There are no encoding considerations beyond
those in [RFC3986].
Applications/protocols that use this URI scheme name:
The "turn" URI scheme is intended to be used by applications with
a need to identify a TURN server to be used for NAT traversal.
Petit-Huguenin, et al. Standards Track [Page 5]

RFC 7065 TURN URIs November 2013Appendix A. Examples
Table 1 shows how the <secure>, <port>, and <transport> components
are populated from various URIs. For all these examples, the <host>
component is populated with "example.org".
+---------------------------------+----------+--------+-------------+
| URI | <secure> | <port> | <transport> |
+---------------------------------+----------+--------+-------------+
| turn:example.org | false | | |
| turns:example.org | true | | |
| turn:example.org:8000 | false | 8000 | |
| turn:example.org?transport=udp | false | | UDP |
| turn:example.org?transport=tcp | false | | TCP |
| turns:example.org?transport=tcp | true | | TLS |
+---------------------------------+----------+--------+-------------+
Table 1
Appendix B. Design Notes
o One recurring comment was to stop using the suffix "s" on the URI
scheme, and to move the secure option to a parameter (e.g.
";proto=tls"). We decided against this idea because the STUN URI
does not have a ";proto=" parameter and we would have lost the
symmetry between the TURN and STUN URIs.
o Following the advice of Section 2.2 of RFC 4395, and because the
TURN URI does not describe a hierarchical structure, the TURN URIs
are opaque URIs.
o <password> is not used in the URIs because it is deprecated
[RFC3986]. <username> and <auth> are not used in the URIs because
they do not guide the resolution mechanism.
o As discussed at IETF 72 in Dublin, there are no generic parameters
in the URI to prevent compatibility issues.
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